1. Field of the Invention
The present invention relates generally to an instrumentation amplifier. More particularly, the present invention relates to a design for a monolithic two op-amp instrumentation amplifier which extends the range of high common mode rejection to frequencies an order of magnitude higher than achievable by conventional means.
2. State of the Art
The primary function of an instrumentation amplifier is to provide gain and to immunize signals from undesirable disturbances. To do this, a good instrumentation amplifier must provide a high common mode rejection ratio (CMRR), high power supply rejection ratio (PSRR), high input impedance and high gain accuracy. While the DC CMRR is often used as a figure of merit for an instrumentation amplifier, many applications (i.e., bridge sensors) also require rejection of higher frequency common mode components (e.g., line frequency and harmonics).
Traditionally, three op-amp instrumentation amplifiers have been used for applications where AC CMRR performance is critical. Two op-amp topology, which is also widely used, provides a significant decrease in price and die area, but conventionally provides inferior CMRR performance over frequency range.
U.S. Pat. No. 5,258,723 discloses one technique of achieving a single supply instrumentation amplifier having high AC CMRR specifically adapted for use with a zirconium-dioxide oxygen sensor for automotive applications.
FIG. 1 shows a graph of the CMRR versus frequency for several prior art instrumentation amplifiers. To effectively reject line noise (at 60 Hz) and its harmonics, it is desirable that the CMRR stay above 90 dB to at least 180 Hz. Grid line 16 represents 200 Hz. For each of the prior art instrumentation amplifiers shown, the CMRR drops below 90 dB before 100 Hz and, thus, does not effectively reject the line noise harmonics. Therefore, it would be desirable to provide a low power, low cost, general purpose instrumentation amplifier which maintains its high CMRR at higher frequencies, yet does not use excessive die area.
It is an object of the invention to provide a technique for a low cost, low power two op-amp instrumentation amplifier, which extends the range of high common mode rejection ratio to frequencies an order of magnitude higher than achievable by conventional means.
It is another object of the invention to provide a new approach for designing the first op-amp of an instrumentation amplifier, wherein the frequency response of an amplifier is manipulated such that the dominant pole is pushed out to higher frequencies than is possible using traditional methods, while maintaining open loop gain and closed loop stability.
It is yet another object of the invention to provide a design for a first op-amp of an instrumentation amplifier which includes a first path which dominates the frequency response of the first op-amp at low frequencies, and a second path which dominates the frequency response of the first op-amp at high frequencies.
The present invention provides a design for a low power instrumentation amplifier which extends the high CMRR range to frequencies previously only achievable with the large bandwidth and higher quiescent current required by prior art instrumentation amplifiers.
According to one embodiment of the invention, a first op-amp of a two op-amp instrumentation amplifier includes first and second gain stages coupled in series, and a third gain stage coupled in parallel to the series combination of the first and second gain stages. The output of the series combination is added to the output of the third gain stage at a summing junction. By including two gain stages in series, the frequency response of the first op-amp is characterized by two poles to allow the open loop gain of the instrumentation amplifier to drop off rapidly after the desired frequency, thereby minimizing the required unity gain bandwidth. By including a third gain stage as a feedforward path, the frequency response of the first op-amp is further characterized by a zero to maintain the stability of the frequency response by canceling the effects of one of the poles at high frequencies. This configuration of gain stages provides the desired result of achieving a low cost, low power, two op-amp instrumentation amplifier with high CMRR.